on board diagnostics is jus an easier way to help u diagnose engine problems and gives all u nature lovers a break b/c a car that runs in good condition will burn fuel better and will put out less air pollutants. what it baseically does is it monitors everything thats goin on under your hood that makes up the engines management system and can notice a problem b4 u would even notice it. for example, say ohh i dunno u have a bad spark plug, itll cause the engine to misfire causing ur catalytic converter to get fawked up and leave u with an expensive bill. What itll do is it turn on your check engine light and to let u know to bring it to a mechanic, which hell go, slap on his expensive scan tool to ur diagnostic port in ur car and get the trouble code from ur cars computer.... hope that helped ya out